The invention relates to a circuit arrangement for generating square pulses, having an edge-triggered flip-flop and at least one comparator, whose output is connected to the trigger input of the flip-flop, and an energy-storing element, which is charged in alternation as a function of the switching state of the flip-flop, and at least one switching threshold resistor is connected in series with the energy-storing element, at which resistor a voltage generated by the current flowing through the energy-storing element drops, which voltage is fed to the signal input of the comparator.
Using such a circuit arrangement to generate square pulses is known. The known circuit is used for instance to measure the field intensity of a magnetic field. A magnetic field probe, which is embodied as an inductive resistor and represents the energy-storing element, is placed in the magnetic field to be measured. The magnetic field probe is embodied such that it is brought to saturation by the magnetic field to be measured and the magnetic field generated by the current. As long as no external magnetic field acts on the magnetic field probe, or in other words the magnetic field to be measured is zero, the magnetic field probe has an electrical behavior that, in terms of an electric current flowing through it, is independent of the direction of the current. The square pulses generated by the circuit arrangement, as a result, have a pulse-duty factor of 1:1.
If an external magnetic field acts on the magnetic field probe, that is, if the magnetic field to be measured is no longer zero, then magnetic field probe reaches saturation earlier in one direction than in the other. With respect to an electric current flowing through it, its behavior is therefore no longer dependent on the direction of the electric current. As a result, the pulse-duty factor of the square pulses changes. Hence the pulse-duty factor of the square pulses represents a measure of the magnetic field acting on the magnetic field probe.
A known circuit arrangement is shown in FIG. 3. In the known circuit arrangement, two comparators are provided, whose outputs are fed to an AND gate 22, whose output is connected to the trigger input of the flip-flop 21. The signal inputs of the comparators are each connected to a different end of the energy store, that is, of the magnetic field probe. The energy store is connected between the two outputs of the flip-flop. Thus depending on the position of the flip-flop, current flows in a different direction through the energy store. Between the outputs of the flip-flop and the energy store, respective switching threshold resistors are connected. The junction points of the switching threshold resistors and the energy store are each connected to the signal input of a comparator. The two reference inputs of the comparators are connected to one another, so that the same reference voltage is present at both comparators.
At the output of the flip-flop at which there was no output voltage, there is now an output voltage after a switchover of the flip-flop, and there is no longer an output voltage at the other output. By means of the energy stored in the coil, the original current flow is, however, maintained. As a result, the potential at the signal input of the applicable comparator drops below the switching threshold. As a consequence, the voltage at the output of the applicable comparator becomes zero. As a result, the output of the AND gate also becomes zero, so that the voltage at the signal input of the applicable comparator again reaches the switching threshold, causing the comparator again to output an output signal, and the AND gate is switched through. By means of the edge occurring upon switching of the AND gate, the flip-flop is triggered again, so that it switches over once again, and the process just described is repeated. The circuit is dimensioned such that it oscillates at a frequency of approximately 350 kHz.
However, the known circuit has the disadvantage that the tolerances of the switching threshold resistors and the tolerances of the switching thresholds of the comparators affect the pulse-duty factor. Moreover, different delay times of the comparators adversely affect the symmetry of the circuit arrangement. The transit time of the AND gate also adversely affects the resolution achieved with the circuit. Furthermore, additional costs result from the use of an AND gate.
It is an object of the invention to provide a circuit arrangement for generating square pulses of the above-described kind in which th influence of tolerance in the components is lessened.
It is another object of the present invention to provide an improved compensation current sensor for current flowing in an electrically conducting element with a controller, which includes the circuit arrangement for generating square pulses according to the invention.
According to the invention, a circuit arrangement for generating square pulses, having an edge-triggered flip-flop and at least one comparator, whose output is connected to the trigger input of the flip-flop, and an energy-storing element, which is charged in alternation as a function of the switching state of the flip-flop, and at least one switching threshold resistor is connected in series with the energy-storing element, at which resistor a voltage generated by the current flowing through the energy-storing element drops, which voltage is fed to the signal input of the comparator, is characterized in that the energy-storing element is disposed in the transverse branch of a bridge, in each of the four bridge segments of which a respective switch is disposed, and the switches are each connected in pairs in crossover fashion by the flip-flop, so that the current flow in the transverse branch is reversible, and that the bridge is connected in series with the switching threshold resistor, and the junction point of the bridge to the switching threshold resistor is connected to the signal input of the comparator.
By means of the arrangement according to the invention, it is advantageously unnecessary to have two switching threshold resistors. Not only is the space required reduced, but by using only one switching threshold resistor, asymmetries are avoided. This is because the current flowing through the energy store always flows through the same switching threshold resistor, and thus the tolerance of the switching threshold resistor has the same effect in both switching states of the flip-flop.
The same is true for a tolerance of the comparator. Since only one comparator is used, tolerances in terms of the switching threshold and the transit time in both switching states of the flip-flop have the same effect. Moreover, both space and expense are saved by using only one comparator.
Since in the circuit arrangement of the invention an AND gate is no longer necessary, the transit times caused by such an element cannot have any effect. Omitting the AND gate advantageously also saves space.
An embodiment of the invention in which the energy-storing element is an inductive resistor has proved especially advantageous. Because the energy-storing element is an inductive resistor, it can be embodied as a magnetic field probe, as provided in a further particular embodiment of the invention. When the inductive resistor is embodied as a magnetic field probe, the circuit arrangement of the invention can especially advantageously be used for measuring a magnetic field.
It is especially advantageous if the magnetic field probe is used to detect the magnetic field of a core of a compensation current sensor, as is contemplated in a further particular embodiment of the invention. By using the circuit arrangement of the invention in a compensation current sensor, the accuracy of the compensation current sensor can be improved in a simple way.
In an advantageous feature of the invention, the comparator is embodied as a digital gate. As a result, the circuit can be produced quite economically. An AND gate could for instance be used as the digital gate. However, it has also proved advantageous to use an analog comparator, which as its output signals furnishes digital signals that are simple to process further.
In a further particular embodiment of the invention, it is advantageously provided that the switches are MOSFETs, of which two are triggered directly and two are triggered via inverters from the outputs of the flip-flop. Embodying the switches as MOSFETs makes it simple to realize the circuit arrangement of the invention. Moreover, triggering of the switches is made simpler. In addition, the switches can be triggered quite precisely.
It has furthermore been found that it is especially advantageous if in the transverse branch of the bridge, a series resistor is connected in series with the energy-storing element. By means of the series resistor, the sensitivity of the circuit arrangement can be adjusted in a simple way.
Further details, characteristics and advantages of the present invention will become apparent from the ensuing description of a particular exemplary embodiment in conjunction with the drawing.